Do I need a handheld light meter?
A partial review of Sekonic L-308S light meter
It's a common question. Not everyone does, the camera has a good meter. But handheld is an extra step, and not everyone will bother with it. Point&shooters certainly won't bother looking at any meter. A handheld incident meter can be a real good thing, but it depends... IMO, we don't see very many using a handheld meter outdoors (because an incident meter is used at the subject, NOT at the camera). But many do find them necessary and extremely useful for multiple flash indoors. The camera meter generally cannot meter flash (excepting for one hot shoe TTL flash). To use flash in a studio situation (which that lighting is normally a few multiple manual flashes), yes, the handheld meter is the necessary norm, to KNOW what your lights are doing, and to help set them up to exactly do it. That's an extremely different situation than the snapshot at the beach or of the mountain outdoors.
There can be exceptions, but for general photos outdoors without flash, a handheld meter becomes greatly more optional. We all already use a light meter, a very good one is in the camera. However, it is a reflected meter (which is adversely affected by subject colors), and the camera cannot meter manual flash. These are limitations. We can learn compensation to use the reflected meter in the camera, and trial and error to handle one hot shoe flash, and get good results. But a better handheld meter normally can meter incident or reflected light, and can meter flash or continuous light, and do it very well. An incident meter (metering principles) certainly can have strong advantages.
IMO, the question basically boils down to being about using an incident meter instead of a reflected meter.
Reflected meters are aimed at the subject from the camera, and meters the light that the subject's colors reflect. The average of any reflected metered area is set to be about middle gray, 12.5% specifically for meters (gray cards are often 18%, which is 1/2 EV difference). Kodak routinely said if metering on their 18% gray card, to increase exposure 1/2 stop.
Reflected meters can only try to place the scene's average tone near the middle.
A white dress or background reflects a lot of light, and reads high, so by moving this down to the middle, the meter underexposes the picture.
A black dress or background reflects little light, and reads low, so by moving this up to the middle, the meter overexposes the picture.
The expected reflected goal is that all results come out about middle gray brightness, not too dark, not too bright. This is all the meter can do, it just sees a blob of light, and does not know what the subject is, or how it should look. Fortunately, many typical scenes contain a random mix of dark and light colors that will probably average out about middle gray tone (not necessarily literally meaning gray, it might be all green or blue. Just meaning middle tone brightness, which is frequently referred to as middle gray, from the B&W days I suppose). Then the middle tone result can often be about correct (and the purpose of the reflected meter is to put any result in the middle). When the scene is otherwise, we can compensate it, to correct it to come out as we want, brighter or darker, as needed.
A Spot Meter necessarily is a reflected meter, and then we need to know and consider the reflectivity of that selected spot if it is to have meaning. It does NOT AT ALL make the spot be "correct exposure", it just makes it come out about middle tone. It could be correct if you selected a middle gray tone spot. But if spot metering on a Caucasian face, you better open up about one more stop, to make it lighter.
Incident meters are the reverse, aimed at the camera from the subject, which reads the light incident on the subject by reading the light source directly. It meters the actual light incident on the subject, at the subjects location, instead of the light reflected from the subject. Incident metering does not see the subject, it instead meters the actual light, which makes it independent of the variations of colors reflected by the subject. Incident works for whatever is there in that light. So then any subject's tone, be its colors light, dark, or middle, is shown as it is. Which is kind of a big deal. Incident meters have the accuracy that newbies imagine their reflected meters ought to have (but can't, simply due to different colors reflecting light in different degrees). This is not possibly done at the camera, incident meters meter the light incident on the subject (actually from the subjects position, which is all important due to the Inverse Square Law). However, it can be less convenient to meter at the subjects position instead of just pointing the camera at it. Incident metering is NOT point&shoot.
IMO, the subject of handheld meters is primarily about incident metering. The camera meter can do as well or better than any handheld reflected meter, but incident meters have a practical accuracy only dreamed of by reflected meters. And incident meters can meter flash.
Incident meters are accurate, and are simply wonderful in the studio, but metering from the subjects location is generally more awkward to use out in the field.
Incident metering, and/or Manual flash, are probably about the only reasons to choose a handheld meter. Very big reasons however. See a closer look at incident metering differences. Your style of metering at the camera has to change though. Incident meters are used AT THE SUBJECT, to meter the actual incident light on the subject.
The incident meter may be awkward outside in the world, since it is metered at the subject location (in the subjects light) instead of from the camera location. However, Incident metering is wonderful in the studio, because it will meter the flash, and the subject location is handily available, and incident metering is simply more accurate (because it is independent of the reflected subject colors). Lighting can be set up before the subject arrives, the subject is unnecessary for metering, only the light at their location matters. Metering each individual flash is used to set that flash power level, which sets a desired lighting ratio. It is good for the entire session, and same setup is easily exactly duplicated next time too. (One exception: a hair light really has to be adjusted by eye, on the subject).
Manually metering the flash is only done for Manual flash mode M and Manual camera mode M, and with Auto ISO OFF. Manual flash cannot change its power level, nor can it react to ISO or f/stop changes. Manual means manual, which is not point&shoot. Only your control can change anything, and it will help to think a bit about what you're doing. But then flash exposure will not change during the session unless you change something (assuming you allow the flash full recycle time, etc).
In contrast to Manual operation, camera meters do meter TTL preflash, and do change TTL power levels of a TTL controlled flash to match the current camera settings. But manually metering TTL flash cannot control a TTL flash. The camera meter is controlling TTL, and you use Flash Compensation to help TTL flash exposure. (And FWIW, the manual meter will only see the weak TTL preflash anyway).
Flash power level: The flash meter does NOT first directly tell you what power level to set on the manual flash. It just has to see a flash first. The use is to first set some guessed flash level (the second time in a similar setup, you should have a good idea what it should be), and you set desired ISO and shutter speed on the meter. Then with an incident meter, you meter the flash at the subjects position (due to the Inverse Square Law effect at different distances from the flash). The meter reading then tells you the f/stop to be set for a correct flash exposure. If that is not the f/stop you want to use, then you change the flash power level accordingly, or maybe change the aperture or ISO by the corresponding difference amount. What you need to know is that each change to double or half power level is ± 1 EV. Then you can meter again for verification of your adjustment, and when as desired, then take your pictures at these correct settings. If you metered properly (without a blunder), it will be accurate.
Shutter speed: Flash exposure itself is not affected by shutter speed (the flash duration is shorter than the shutter duration — the shutter merely needs to be open to pass the flash). However most flash pictures also involve some degree of ambient light present, so flash pictures often also have an added ambient light contribution, so shutter speed cannot be ignored. A slow shutter speed does not affect flash exposure, but it certainly does affect the ambient light exposure (if present in any significant degree), so a slow shutter speed effect may be seen on the meter and on the camera. So because of ambient, the flash meter should be set to the same shutter speed as the camera will use. Depending on our goal, we can choose a show shutter speed to allow the ambient in, or we can choose a faster shutter speed to keep it out.
For your important portrait lighting set up (if you are using flash), I suggest a low ISO at ISO 100 and the Maximum shutter sync speed (like 1/200 second), and a modestly stopped down aperture (like f/5.6 to f/11, assuming a full frame camera), all done to keep ambient out, and then flash power will get the job done. Take a first test shot of your setup with flashes turned off, hopefully will be a black frame, to verify there is no significant ambient contribution.
Results: Specifically for example, the suggested ISO 100 and f/8 and camera mode M with shutter speed of Maximum Sync Speed (typically around 1/200 second) will make the indoor room ambient level be insignificant compared to the flash intensity. This shut-out even includes the flashes modeling lights (which you may have to see to believe). Keeping out the ambient this way is a big benefit for portraits, in that the ambient light does not interfere with flash White Balance or intended Lighting Ratio, and will also provide the portrait with ample Depth of Field (see metering the setup below).
In contrast again, TTL flash indoors using Auto ISO will first choose camera settings suitable for the dim indoor ambient — meaning highest ISO and slowest usable shutter speed are required indoors. This makes the ambient light be the dominant light, and balanced TTL flash will be at lower fill levels. Consider selecting Incandescent white balance then, but which still is a mixed white balance problem (and a high ISO problem, etc.) But if you care about white balance, you can turn Auto ISO Off, and choose low ISO, and select camera M mode and a fast shutter speed, even with TTL flash. TTL still will again adjust flash power for those settings. TTL power for bounce flash may need ISO 400, but that's still lower.
As a very quick spot check of meter accuracy, bright sun should meter near EV 15 at ISO 100. This EV 15 is f/16 at 1/125, or Sunny 16 would say f/16 at 1/100 second (both at ISO 100). Or equivalent exposures, f/11 at 1/250 or 1/200 second, or f/8 at 1/500 or 1/400 second, etc. Maybe not necessarily precisely EV 15, bright sun could often seen at times be a bit less bright, as this will vary slightly with air clarity and angle, haze, time of day or season, latitude, etc. The meter indicates the precise value.
Other than TTL flash, the camera meter does Not meter manual flash. We can easily adjust one hot shoe flash in manual flash mode, done trial and error by eye on the rear LCD preview image, or with the histogram. So people may tell you "nah, you don't need no stinkin' meter", but they only know about their one flash situation. But what about multiple studio flash? The idea is to control every flash unit, to know each one is doing exactly what you want. Not everyone uses multiple studio lights, but if you do, you will certainly appreciate the handheld meter (you already know this, really no other way to do it consistently). Otherwise, how will you duplicate your last sessions lighting setup again? Lighting is really no longer merely about exposure, but is about lighting ratio, the relative power of each flash. Lighting ratio is very important, and the handheld meter makes it be easy to set each light to an exact value, both easy to do it, and in particular, to know what you are doing, and to be able to repeat it next time. The camera commander becomes insufficient, it does control two individual lights, but our setup may use four lights. And the handheld meter is incident metering where the commander is reflected. These are big differences, and a big deal.
This all seems pretty obvious, but usually none of it is ever mentioned in discussions about if handheld meter is useful. Many people may not understand why a handheld meter might be used. The camera meter is excellent, it meters reflective metering very well, surely it is the best reflective answer. The only problem is that it is reflective, fooled by subjects reflected colors. This handheld meter purpose is normally about incident metering, and/or about metering flash. There may be other minor considerations, but if flash or incident metering advantages were your concerns, then the handheld meter is the solution. If you only want another reflected meter, I'd think about that longer. If you use reflected metering, the camera meter is hard to beat (but there is still flash). It has advantages (internal meter sees same angle of view as the lens), and it can do Spot too. But yes, other features are available, notably incident and flash metering.
This Sekonic L-308S meter, was the simpler and less expensive Sekonic model. The current version of it is Sekonic L-308X-U Flashmate Light Meter $219 (USD). No frills, but very nice and all the basics. It can meter incident or reflected light, and it can meter flash or continuous ambient. IMO for studio use, it will do all that needs to be done. The differences I detect in the newer L-308X specs is addition of a ± 1 EV calibration compensation, a range to EV -6 instead of EV -5, addition of cine frame rate (I suppose to be a lower limit on cine shutter speed), and a longer 3 year warranty now.
Incident and reflected metering results likely can vary slightly, because incident is directly reading the actual light source incident on the subject, and reflected is reading the reflection from whatever 40 degree wide view and colors that it sees. Two different situations, and this is why we use incident (but a camera cannot, because it does meter at the subjects location).
I also have a Sekonic L-358 (no longer in production). The current replacement is the L-478D, which has a touch screen. Sekonic LiteMaster Pro L-478D-U Light Meter $369 (USD).
The L-358 is very nice, it's been a classic, and I like its wheel better than the up/down buttons. If I used it outdoors, I'd like its aperture preferred option too. To meter reflected light, you have to swap out its dome with a flat one (which becomes a 54 degree field). But I don't need its frills, which are simply wasted on me. I've learned to use the camera meter outdoors, and only use a handheld meter for multiple studio flash, and the L-308S is not missing anything for that. In my use, I don't see that the L-358 was worth $150 more than the L-308. The L-308 simply works wonderfully.
For metering reflected light, this L-358 model replaces the dome with a flat disk provided instead. The L-308 model has a disk for flat subjects, but for reflected, it removes the dome, to use the bare sensor instead. But frankly, if we have incident metering, in the studio, we use incident. And incident is ideal for metering flash (at the subject).
Use of a light meter:
- Incident metering: — Aim dome towards camera from subject's position. The meter must be in the same light as is on the subject, and if for flash, must be at same distance from the light as the subject (one portrait idea is to hold it under the subjects chin, to meter the frontal LIGHT on the subject). Don't block the light with your body.
- Reflected metering: — Slide L-308S dome to the side to reveal the bare sensor, and aim it at subject from camera position. The reflective field width is 40 degrees for the L-308S. The L-358 replaces the dome with the flat disk for reflected, and is a 54 degree field. In contrast, the camera meter is also a reflected meter, but it sees the width of the view that the current camera lens sees, and many cameras also provide a spot metering option (spot metering is also reflected metering).
- Incident works in any light at the subjects location, metering the Sun, etc. But flash mode must be Incident and must be triggered. The meter has two Flash modes: Cord or Cordless (depending on how you trigger the flash to be metered). Cord mode connects a PC cord from the light to the meter, and a button on meter triggers the flash and meters it. Cordless mode reads the first flash burst seen in the next 90 seconds, regardless how it was triggered. An incident meter does not see the subject or any reflected colors, but instead simply and directly meters the light that is incident on the subject. That is a plus (no variations from reflected colors).
- Ambient — use Ambient mode (mode is shown in LCD). This is reflected light seen in a reflected meter, and incident light seen in an incident meter.
- As for all meters, read instructions. Sekonic manuals are all online at Sekonic product pages, and the Support page.
I use the handheld incident meter mostly for multiple flash studio, used a few minutes each session to setup the lights. It's great to measure each flash individually, to set the power level of each light right, to know what they are doing, and that they are doing what we want them to do. And then we can repeat the setup easily again next time. Incident meters are wonderful (better accuracy than reflected meters). However, I've also used reflected meters forever, so ambient exposure, or flash compensation, with the camera meter is no mystery, still convenient for everything else (other than multiple manual flash). However, beginners will need to learn that reflected meters are just a guide, affected by what you point them at, and which don't always indicate precise proper exposure. The L-308S is also an incident meter, so exposure accuracy is not much issue then, but incident meters do have to meter from the subjects position. Experience with both types can teach a few things about metering exposure.
The Sekonic L-308S
L-308S Technical Data (from the manual)
Type • Digital exposure meter for ambient and flash light
Light receiving method • Incident light and reflected light
Light receiving section
• Incident light: Lumisphere, Lumidisc
• Reflected light: Lens (light receiving angle of 40°)
Light receptor • Silicon photo diode
Measuring modes
• Ambient light:
Shutter priority metering
EV metering
• Flash light:
With synchro cord
Without synchro cord
Measuring range • Ambient light: EV0 to EV19.9
(ISO 100) • Flash: F1.4 to F90.9
Repeat accuracy • ± 0.1 EV or less
Calibration constant
• Incident light metering: C = 340 (Lumisphere), C =250 (Lumidisc)
• Reflected light metering: K = 12.5
Display range
• Film speed: ISO 3 to 8000 (1/3 step)
• Shutter speed
Ambient light: 60 sec to 1/8000 sec (in 1, 1/2 or 1/3 step)
Cine speed (f/s): 8, 12, 16, 18, 24, 25, 30, 32, 64, 128
(shutter angle: 180 degrees)
Flash: 1 sec to 1/500 sec (in 1, 1/2 or 1/3 steps) and 1/75, 1/80, 1/90, 1/100 sec
• Aperture value: F0.5 to F90.9 (in 1, 1/2 or 1/3 steps)
• EV (exposure value): EV - 5 to EV26.2 (in 1/10 step)
Other functions • Out-of-range indication: E.u (underexposure) and E.o (overexposure) warnings
• Battery check indication with 3 level status icon
• Auto power OFF (approx 4 min after last operation)
• Custom setting
Battery used • One Type-AA 1.5-volt battery (alkaline, manganese or lithium, but Not Ni-Cd or Ni-MH)
Operating temperature range • 0°C to +40°C
Storage temperature range • -20°C to +60°C
Dimensions • Approx 63W x 110H x 22D mm
Weight • Approx 95g (battery included)
Standard accessories • Soft case, strap, Lumidisc, soft case for Lumidisc, synchro terminal cap, type-AA 1.5-volt battery
Features and specifications are subject to change without notice.
The L-308S has all the basics, and works great. Minimal in that it doesn't have the frills, but it is first class on the basics we actually need. I have no question about its accuracy (Specifications list a slightly less extreme range, but same +/- 0.1 EV accuracy as top of the line). It just does not have the extras seen in more expensive models. It simply meters the lights, and does what it needs to do, and does it very well, but no frills. I call them added frills, of no use to me, but perhaps workaround is an experience/skill thing. Others may consider them more essential, but simplicity and ease of use and lower cost are major features too.
Among these missing frills NOT in this L-308S model are these features:
- This one may be a bigger deal to some people — the L-308S model is shutter preferred readings only. Which is absolutely perfect for flash, this is the only way flash can work, so this is no issue at all for indoor flash. Because flash is independent of shutter speed, basically any usable shutter speed results in the same flash exposure (meaning, there is no regular concept of Equivalent Exposures in flash mode, at least Not by using shutter speed). Shutter speed of course must not exceed the camera shutters Maximum Sync Speed (which is a shutter issue, not a flash issue). The sync is to ensure the shutter is open to pass the flash.
Still we might prefer aperture preferred metering for ambient. We can always do that conversion in our heads (simple equivalent exposures). But easier, the L-308S meter can show the conversion too. Just take a reading, which is shutter preferred However if in Ambient mode, then after that, watching that reading as you change the shutter speed setting, the corresponding aperture will track it and you easily see the other equivalent exposures. So maybe we can't specify the aperture in advance, but we can easily find it after. However in flash mode, then changing shutter speed will zero a previous reading, which is as it must be... because flash is independent of shutter speed (flash still needs the same aperture it metered). Flash is Not an "equivalent exposure", flash just tells you which aperture. With flash, that is all you want to know, shutter speed does not matter (if not exceeding Maximum Sync Speed). So, this is absolutely zero issue for metering flash.
(FWIW, a different subject, but HSS flash is affected by shutter speed and can use Equivalent Exposures.)
- Any TTL flash cannot be metered with a handheld meter due the TTL preflash interfering. But the big thing of course is that TTL does its own metering and control regardless of what any handheld meter might see and think.
- But regular manual flash (speedlights or studio flash, in Manual flash mode) is metered with regular flash mode (meaning Cord or Cordless on the light meter). We do set the shutter speed, which is the time duration that the meter reads the Ambient light (if any is contributing). Flash exposure itself is generally not affected by shutter speed (but using maximum shutter sync speed seems important in studio sessions to suppress any stray ambient). The meter meters the flash power at the subject and tells us the aperture, maybe tells us f/5.6, and we can set the camera to f/5.6. Note that f/8 requires a stronger flash power than f/5.6... we must open aperture to f/5.6 for the weaker f/5.6 level flash.
- The L-308S model can do either incident and reflected metering, but this model does Not do Spot metering. All Spot metering is only reflected (and the handhelds that do it likely also need an added accessory). If you meter the incident light itself, spot has no meaning. The camera can do reflected Spot metering (but not of the flash, the camera's flash metering does not use Spot). If incident metering is available, you ought to try it. Incident metering is wonderful, it meters the source light, and is independent of the reflection of the colors of the subject. Metering the actual light level makes Spot metering unnecessary, your picture will be fine. Most users don't know how to use Spot Metering anyway.
- Another frill is the ability to store and compare two readings, to tell us computed difference in two readings (like main and fill lights for lighting ratio). How much difference is there between f/10 and f/6.3? This difference option will compute that. However, the L-308S (and others) will meter in tenth stops, and then the difference in two lights is totally trivial to compare in our heads, then no issue at all, much faster than messing with more menus and buttons. On the L308S, hold the Mode button while turning meter on, to select and set Full, half, or third stop metering. Full stop metering mode also shows tenth stops. See the advantage of tenth stops below, it is much easier. Tenths are very helpful for multiple flash, to easily know the lighting ratio.
- The L-308S shutter speed range is 1 second to 1/500 second for flash mode (ambient mode is 60 seconds to 1/8000 second). It has a Custom Setting to set it to display full stops (which also shows tenth stops), or third stops or half stops. Bigger meter models will meter flash up to 1/1000 second, but it's a very rare camera shutter than can use that with flash. Most camera shutters in flash mode are limited to a slower maximum shutter sync speed anyway, but FP HSS flash shutter speed might be used faster, and since HSS is continuous light, equivalent exposures work (same as for sunlight). For HSS continuous light, you can use any appropriate equivalent exposure to convert it faster... 1/500 f/8 is 1/2000 f/4, etc.
- The L-308S does Not show percentage of manual fill flash compared to total exposure with ambient. Higher models do. However to me, knowing that flash is 20% or 30% of the total seems not the best way to think of it (those are coarse steps). We normally measure lighting ratio in stops (with 1/3 stop precision), and we can see the increase when we turn the manual flash on. If using TTL flash, we already know ratio, as Flash Compensation. If you think you need to know percentage, see a calculator to compute flash percent from TTL compensation EV.
- The L-308S also is Not designed to accept a PocketWizard addon to trigger metering remotely (using radio from meter to flash instead of a PC sync cord). I can't imagine many who are interested in saving a little on a meter are going to buy a full set of Pocketwizards anyway, so your concern may vary. See the way to connect any meter to any radio trigger. This feature may be more important to professional users (when time is money), but it is unimportant to me, since I use optical slaves instead (studio lights already have the slave, and it works great indoors). My hair light is too high to reach (to plug in metering sync cord), so I just use a small three step ladder to do it (used also to aim it). No big deal, I only use the ladder once during the session. IMO, this too is easier than the alternative. 😊
- The L-308S does Not provide a way to "calibrate" it. No meter offers calibration, but bigger models typically do add a simple compensation offset (which depends entirely on the user to do any calibration). The L308S offers no compensation feature. Maybe a plus, perhaps one less worry, but I use the actual meter reading, it seems very accurate. A camera exposure of course confirms if it is right. That calibration concept is not easy to grasp, and some understanding would seem necessary. In contrast, the simplest notion imagines a gray card value ought to be at middle of the digital histogram. But the digital histogram is an entirely different concept than film. 18% is 18%, not 50%. The ONLY reason 18% seems to approach 50% in the histogram is because (purely coincidentally, entirely unrelated to exposure), gamma does make 18% come out about 46% gamma in the histogram. But users assume 18% has become 50% somehow, and it must be midpoint (whatever they imagine that means). Our adjusting 46% to read as 50% is only a small error (about 8%), photographers can easily deal with it, but it would be an error to do so. The meanings of "middle" are different, and gamma has boosted the actual midpoint to 73% anyway. Understand that the cameras Contrast and White Balance curves are also shifting the histogram, so adjusting 46% to read 46% is not the best plan either. The best plan (if actually necessary) is to correct for many consistently seen multiple readings, not just some one reading.
To easily see this, tweak a test exposure of something bright to exactly line up almost at the right 255 edge of the (RGB) histogram. Then intentionally underexpose exactly one stop. That's 50%, and yes, linear data ought to now line up at digital 128. But the gamma histogram is not linear, and it is NOT at 128, because gamma computes half to be at 186 (73%). It will be around 3/4 scale, NOT 1/2 scale, but it varies, because the camera is also doing other things too, white balance and contrast and Vivid, etc. Repeat this shot with Daylight and then Incandescent White balance, and compare the curves. The data edge probably won't be 186 in either, and ballpark is the best hope. Do you really want to calibrate on this? The histogram is NOT a light meter, and the digital histogram is not linear (it is gamma encoded).
If in dozens of different situations, the meter is always consistently off a little, and exactly predictable, that would be a reason to adust it. But certainly NOT based on the bad logic of ONE reading on a gray card! If you do want the meter to read 1/3 stop higher, you can just enter its ISO to be 1/3 stop lower than it is. But my bet is that the meter works great as it is, so be careful not to screw it up. My own personal metering variations are my problem (caused by me), and I'm guessing your problem too. There is some skill involved to learn to meter more consistently, especially for studio flash. The meter is better than I am.
Perhaps that seems a long list of omissions, and your needs may be different, and the L-358 does add these frills (which I have never used with it). Speaking of a studio situation, IMO it's just clutter, none of which helps me to meter the lights. After using the L-308S several years (for studio flash), I don't feel I'm missing anything. What the meter does do is to meter the light, very well. The L-308S does exactly what I want it to do, no more, no less. Simplicity and ease of use seems a major feature. I have both, and normally use the L-308S. However, the Sekonic L478D is about $150 more, about like the L358 except with a touch screen. It does include these other features. You can study the user manuals, which are available on the Sekonic product pages.
The L-308S is simple to use, no complex menus or features. It just measures the light. The meter has a mode for continuous light (the sun icon), and it has two modes for flash, called corded and cordless. Corded means using a PC sync cord to trigger the flash as described in the example next below. Or Cordless uses no PC cord, but instead after pushing its button, it waits 90 seconds to see a flash fire, any way you can trigger it, and then it meters that flash.
It meters incident light with the dome aimed at the camera from the subjects position, and it meters reflected light with the dome aimed at the subject from the camera position. The Lumisphere (dome shown) is used for normal 3D subjects, and there is a flat "Lumidisc" used for metering flat 2D copy (paper, paintings, etc). The dome slides away from its lens only for purpose of swapping these two. The meter does not necessarily meter the same view that the lens will see. The L-308S meter's reflected field of view is 40 degrees, which field width matches a 50 mm FX lens, or a 32 mm DX lens. The L-308S uses one AA battery, which seems handy.
A light meter is used for real actual old fashioned manual flash, like studio flash units, or the mode called M on the speedlight. Specifically, no commander, no TTL. We cannot meter TTL flash, which is pointless anyway (since the camera is going to control it), and all we will meter is the weak TTL preflash. And the Commander firing all its flashed commands is very bad news too (some ifs and buts on the AWL page 2 here). The light meter is for Manual flash.
Metering a studio setup with multiple flash
To illustrate the common way a handheld meter would be used for multiple manual lights (studio lights, or could be speedlights):I suggest for bigger cameras (DSLR, etc), to set Manual flash mode and Manual camera M mode, with Low ISO (like ISO 100, and certainly set Auto ISO OFF, manual flash cannot deal with it) and set f/stop up near f/8 (but f/8 will be excessive diffraction for a small compact camera). Camera M mode will allow setting shutter speed to be Maximum Sync Speed (typically around 1/200 second). The point is, ISO 100/ f/8, 1/200 second settings will keep any ambient light level out of the picture. You can take one test picture with flashes off (meaning disable any trigger so they do not flash) but with the necessary ambient light situation left on (with the flash modeling lights on if you will be using them). With flash level set to match the metered camera settings, the test picture result of these settings should be a nearly BLACK frame in the picture taken with flash disabled. That black picture means the ambient light that your eyes easily see is kept out by the settings, at insignificant level to affect your white balance or your lighting ratio, and with ample depth of field for a human portrait subject. That is all good, and the flash has been metered and setup to be perfect at these same settings.
This calculator relates flash power changes by mapping EV to percent change, or percent change to EV, both relative to the previous power level. So a value 50% does NOT mean setting TO 50% power level, here it means a change to 50% of the previous power. Same as EV means a change amount. The 50% of the previous power means like from 1/4 to 1/8 power level is half, and half power is -1 EV. Knowing that half of the previous power is 1 EV is useful when adjusting power level with a meter. And I normally meter multiple flash in tenth stops, for the lighting ratio reason shown next.
See More actual lighting setup detail here. See more about lighting ratios on that same page. Aiming for perhaps 1 EV to 1.33 EV lighting ratio makes a big pleasing difference in color work. Grayscale B&W work might use higher ratios.
From the subject's location, I aim the meter directly at the flash being metered for ratio. I should replace the dome with the flat disk to meter this way (to exclude light from other angles), but it seems unimportant if only one light is on, and I don't bother. The dome definitely is aimed at the camera for any incident reading for camera exposure. The idea of the spherical dome is that it properly factors the cosine response of lights at other angles.
With only one flash turned on at a time, I use the Sekonic L-308S meter by connecting a 15 foot PC sync cord (cord comes with the studio light) between the meter and ONE flash (the only one flash turned on). From under the subjects chin, a button on the meter triggers the flash and meters it. I start with the Main light, and set its power level to read 1/3 or 2/3 stop less than the fstop I want the camera to use (the fill light will add it back, depending on ratio). Then I move the cord to the next of four lights (and turn only it on), and repeat to set its level. This is to set each light to the power I want it to be, so I know what they are doing. Like maybe main near f/8, fill 1.0 or 1.3 stops less than main, background often about same as main, and hair light depends on hair color, maybe 1 stop more than main for black hair, maybe 1 stop less than main for light hair. Then I know what I am doing, and I know what the lights are doing. And another overwhelming advantage, I can easily repeat this exact same setup quickly next time, no guessing. I can do all this (except hair) to be ready and tested before the subject arrives.
The Incident meter is aimed at each light for this, from subjects position (which could be me, sitting there). Background light is metered at background surface, aimed at its light. Hair metered at hair, aimed at its light. Hair light is a starting point, can be close, but hair color may require some trial and error (how it shines in a test picture).
Then I turn on only the two main and fill lights (no background light, no hair light, etc) and meter this exposure for the camera aperture. Meter under subjects chin, aimed at camera here. You can be this test subject, metering under your own chin in the subject position.
Your subject may not have even arrived yet, but you're done and ready now (except for seeing the hair light). Just turn all lights on, move sync cord from meter back to camera, and go at it. For optical slaves, this cord connects to the light that is aimed at the others, which normally is the fill light, back near the camera, behind and above camera in my case. From the camera, fill light is aimed at the subject, but which is also towards the optical slaves on the other lights. This is so simple, and very reliable.
If you are using multiple manual flash, there are not many options... such an handheld meter is the way to meter flash, and to KNOW what you are doing (and to repeat it easily next time).
If you want to work in continuous light, the meter will do that too. As will the camera meter, but incident has strong advantages. So it may be optional, but the handheld metering subject is about incident or reflected. And incident means metering the actual light source directly, exposure independent of the subjects influence, but metered at the subjects location (with only minor exception, clear sky bright sun is mostly the same everywhere in sight).
